This application claims the priority of German patent documents 197 29 517.7, filed Jul. 10, 1997 and 97 122 767.3, filed Dec. 23, 1997, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a solid for storing/releasing nitrogen oxides (NOx) as well as to a nitrogen oxide storage catalyst.
Worldwide, numerous groups are working on the development of catalysts for removing nitrogen from lean-otto-engine exhaust gases. One development direction which is currently pursued intensively concerns the so-called NOx storage catalysts, which are to be used in the exhaust gas of lean-mix-operated otto engines. See Shin'ichi Matsumoto, Catal. Today, 29 (1996) 43/1/ and W. Boegner, M. Kraemer, B. Krutzsch, S. Pischinger, D. Voigtlander, G. Wenninger, F. Wirbeleit, M.S. Brogan, R. J. Brisley, et al., Appl. Catal., B7 (1995) No. 1-2, Pages 153-171. In this case, the nitrogen oxides are stored during lean operation of the engine, and are released again during rich or stoichiometric operating phases and simultaneously converted on a conventional three-way catalyst to N.sub.2. The NOx storage catalyst is correspondingly composed of two material components, the NOx storage component and a solid with a catalytic activity for removing nitrogen (for example, a three-way catalyst). The NOx storage material stores the nitrogen oxides as nitrates. For this purpose, alkaline and earth-alkaline compounds are used as solid constituents.
A decisive problem which currently does not permit the use of the NOx storage catalyst in series is the insufficient resistance of the NOx storage material to sulfurous constituents in the exhaust gas and to high temperatures. Another problem consists of the fact that, because of the oxidation of the NOx to NO.sub.2 required for the nitrate storage, this type of catalyst can be used only above a certain temperature (approximately 200.degree. C.). In the driving test cycle stipulated by law, the low-temperature range (in Europe: Cold start and ECE) therefore cannot be covered.
In the technical literature, Ag on a zeolitic carrier of the MFI type is described as a solid which adsorbs NOx. See, W. Zhang, H. Yahiro, N. Mizuno, J. Izumi and M. Iwamoto, Langmuir, 9 (1993) 2337 and W. Zhang, H. Yahiro, N. Mizuno, M. Iwamoto and J. Izumi, J. Mater. Sci. Lett 12 (1993) No. 15, 1197.
It is an object of the invention to provide an NOx storage material which is suitable for a use in an NOx storage catalyst, and has a sufficient resistance to sulfurous constituents in the exhaust gas.
This goal is achieved by the Ag-containing solid used according to the invention, which can be loaded (among other methods, by adsorption) or unloaded (among other methods, by desorption) with nitrogen oxides by the targeted change of the .lambda.-value (=ratio of the gas constituents acting in an oxidizing to reducing manner) of the gas atmosphere situated above the solid. If .lambda.-values are higher than one, the nitrogen oxides will be stored; if the values are equal to or lower than one, the nitrogen oxides are released.
In addition to Ag-containing compounds (for example, elemental Ag, Ag.sub.2 O, Ag.sub.2 CO.sub.3 or AgOH), the solid used according to the invention, may also contain one or several precious metals (such as Pt, Pd, Rh, Ir, Au, Ru, Os). The Ag-containing and precious-metal-containing compounds can advantageously be applied to microporous carrier substances, such as Al.sub.2 O.sub.3, CeO.sub.2, La-containing CeO.sub.2, ZrO.sub.2, La-containing ZrO.sub.2, SiO.sub.2, TiO.sub.2, Mg-Al mixed oxide, Si-Al mixed oxide, zeolites or a mixture of several of the above-mentioned compounds. In this case, the BET-surface of the carrier substance is between 10 and 1,000 m.sup.2 /g, in particular.
The Ag-containing solid according to the invention, in particular, is suitable for reducing nitrogen oxides in the exhaust gas of lean-operated otto engines and diesel engines.
Because of the described dependence of the storage/release go action on the .lambda.-value of the NOx-containing gas, the Ag-containing solid can ideally be used as the NOx storage component in an NOx storage catalyst. When it is used together with another solid with a catalytic activity for the removal of nitrogen (in the following called a catalyst component), the nitrogen can be removed from high-O.sub.2 gases. In particular, the catalyst component may be one with a three-way function.
The NOx-storing solid according to the invention as the component of an NOx storage catalyst can contribute to meeting future emissions limit values. Its particular distinguishing characteristic is its resistance to sulfur.
Another very advantageous characteristic of the Ag-containing solid used according to the invention is the fact that it stores nitrogen oxides even at very low temperatures. By means of the Ag-containing material, an NOx storage is possible in the temperature range of below 200.degree. C. The release of the stored NOx can subsequently take place at temperatures of above 200.degree. C.
For use in vehicles, the powdery material (catalyst component and NOx storage component on a microporous carrier substance) is applied as a coating to a ceramic or metallic honeycomb body (in the following: geometrical carrier, for a delimitation with respect to the microporous carrier substance). For other applications, it can also be used in shapes differing from the above, for example, as pellets or extrudates.
In this case, NOx storage components and catalyst components can be combined as follows:
Atomic Mixture PA1 Powder Mixture PA1 Layer Arrangement PA1 Structured Arrangement PA1 Impregnation, PA1 sol-gel process, PA1 wet-chemical precipitation, such as hydroxide precipitation; PA1 ion exchange in a zeolite.
NOx storage components and catalyst components are situated side-by-side on a microporous carrier substance, which, in turn, is applied to a geometric carrier.
NOx storage components and catalyst components are in each case situated individually on microporous carrier substances which are applied to a geometric carrier as a powder mixture.
NOx storage components and catalyst components are each situated individually on microporous carrier substances which are arranged in layers and applied to a geometric carrier.
NOx storage components and catalyst components are each situated individually on microporous carrier substances which are each mounted separately from one another on different geometric carriers, for example, in a serial arrangement.
NOx storage components as well as the catalyst components may be applied to the microporous carrier substance, for example, by means of the following conventional processes:
In particularly advantageous embodiments, the Ag-fraction relative to the total mass of Ag-containing solid plus microporous carrier substance amounts to at least 10 ma %.